In the chemical and energy industries pipework is used to both transport and exchange heat to and from fluids. In many enviroments this pipe work becomes `fouled` both by the products of its own corrosion and also by material resulting from corrosion elsewhere which is carried into the system by the fluid flow. The deposits can obstruct both the flow and heat transfer processes requiring that the plant be periodically descaled to return it to its desired performance. A particular example is the accumulation of metal oxide deposits in steamraising plant; these restrict flow in the boiler tubes resulting in increased pressure-drops and in heat transfer impairment and also provide sites for the accumulation of aggressive salts which accelerate tube corrosion.
The construction materials of water-cooled nuclear reactors are corroded by the aqueous coolant and small amounts of their constituent elements are released into the coolant. These constituent elements become neutron activated in the reactor core and are ultimately deposited in the form of their oxides on the vessel and pipework surfaces throughout the coolant circuits, giving rise to large radiation dose rates in the circuit. It is desirable to remove these oxide deposits to reduce the radiation dose rates prior to man access.
Our European Patent Application No. 81.300010.6 describes and claims a process for the removal of deposits consisting essentially of the oxides of one or more transition metals from a surface which process comprises contacting the said surface at a pH below 7.0 with a reagent comprising a one-electron reducing agent which is a low oxidation state transition metal ion in combination with a complexing agent which is thermally stable at the operating pH.
In a preferred aspect of that process the water/steam side of a boiler or a process heat exchanger or the surfaces of a fluid transfer pipe associated with chemical or steam raising plant are desclaed. Thus the transition metal oxide deposits are dissolved and the resulting solution is easily flushed from a complex pipework system, the thermal-hydraulic properties of the system being thereby restored.
In another preferred aspect of that process the cooling system or a component associated with the cooling system of a nuclear reactor, or other contaminated plant items, are decontaminated.
Thus, the radioactive oxides dissolve and a solution is obtained which is suitable for treament by ion exchange to remove both the radioactive ions and the decontaminating chemicals from the system being cleaned. In this preferred process the decontaminating reagents are circulated in the cooling system of the reactor, or contacted with the component to be cleaned in a suitable decontamination facility.
Traditional methods of reactor decontamination employ mixtures of chelating acids which are stable both individually and when mixed in aqueous solution. These acids can therefore be premixed as a solution or slurry and pumped into the circuit to be cleaned. The chemicals to which the present technique relates are, both individually and when mixed, sensitive to both air and the presence of metal surfaces and require special handling techniques if the decontamination process is to be successfully accomplished.
An example of the type of reagent that has previously been used in the decontamination of nuclear reactors is a mixture of citric and oxalic acids. Those chemicals are solids which are stable in air both separately and when mixed together. The mixture can therefore be stored for long periods of time, often years, with no ill effect and it can be dissolved in water in any suitable vessel at any time prior to injection into the reactor or decontamination facility. Stainless steel is the material most commonly used for the preparation and storage of these reagent solutions.
The decontamination reagents described in our European Patent Application No. 81.300010.6 consist of two essential components: a transistion metal ion in a low oxidation state, such as chromium (II) or vanadium (II), and a complexing agent, such as picolinic acid or bipyridyl. The complex formed when the two components are brought together performs the necessary reduction to bring about dissolution of the radioactive oxides. We call these reagents "LOMI" reagents (low oxidation state metal ion reagents).
Although the complexing agent in these reagents is usually a stable chemical, capable of prolonged storage, this does not apply either to the low oxidation state metal ion, in solution or as a solid salt with the appropriate counterion, or the complex formed between the metal ion and the complexing agent. It will be appreciated by those skilled in the art that these reagents are sensitive to oxygen, and must therefore be used under an inert atmosphere. However, we have found that even when oxygen is excluded from these reagents, decomposition of the reducing agent is quite rapid in the presence of materials capable of catalysing the reduction of water by the metal ion. For example, we have found that concentrated solutions of vanadium (II) formate lose much of their reducing ability after only one day in contact with stainless steel. Similarly, dilute solutions of the complex formed between vanadium (II) and picolinic acid rapidly lose their capacity to dissolve oxides when heated in the presence of stainless steel. Other "LOMI" reagents decompose on storage even in vessels made of inert materials such as glass; for example, solutions of vanadium (II), or chromium (II), with the complexing agents ethylenediamine-tetra-acetic acid, or nitrilotriacetic acid, are only stable for a few hours when heated.